For some time, the prior art has recognized the need to minimize static electrical charges in certain laboratory and manufacturing environments. The build-up of static electrical charges on sensitive items, including electronic components such as integrated circuit chips, may result in the catastrophic failure of such items. These highly sensitive items are particularly susceptible to damage from static electrical charges during manufacturing and assembly processes wherein the electrical leads of such items are typically exposed and unshielded. Once installed in the devices for which they are intended, such items are generally less susceptible to damage from static electrical charges, since they are substantially isolated therefrom and frequently grounded in a manner which prevents static electrical charge build-up.
In accordance with contemporary methodology, various means are utilized to mitigate the undesirable effects of static electrical charge build-up in the laboratory or manufacturing environment. For example, the manufacturing and assembly of static sensitive devices is typically conducted in an environmentally controlled facility which is adapted to minimize electrostatic charge build-up. Frequently, the humidity within the facility is maintained within a desirable range such that static charges tend to dissipate, through the moisture ladened air, from the surfaces upon which such charges would otherwise accumulate. Additionally, it is also known to increase the ion content of the air within sensitive areas of such facilities so as to provide a means for neutralizing undesirable static electrical charges. The ions from the air combine with those of an opposite charge which define the undesirable static electrical charges, thus effecting the neutralization of such charges.
In addition to the foregoing, workers in the above-described facilities are generally required to utilize grounding straps upon one of their wrists and/or ankles, and are also often required to wear static charge dissipating clothing, e.g., gowns and/or other articles of clothing, so as to mitigate static charge build-up upon the person and/or their clothing. As used herein, the term wrist strap shall be defined to include such wrist straps, ankle straps, and/or other personnel grounding devices.
One such prior art wrist strap comprises a single line grounding strap which utilizes a single conductive line to bleed static electrical charges away from the user to ground. A resistor is preferably incorporated into the conductive line to ground so as to prevent a direct short to ground if the user should accidentally contact a hot or active electrical conductor. Thus, the inclusion of the resistor within the conductive line substantially enhances the safety of the wrist strap.
In these prior art wrist straps, it is known to continuously monitor the integrity of the conductive path to ground by monitoring the capacitance of the conductive line. In this respect, the capacitance of the conductive line is substantially reduced when the electrical connection of the wrist strap to the user is not adequate, i.e., an open or high resistance connection is present. The integrity of the conductive path may also be monitored in the prior art wrist straps by measuring continuity on an intermediate basis. However, such monitoring requires that the user periodically touch a ground so as to define a current loop through the conductive line, the user, and ground, through which continuity is measured. In the prior art wrist straps, the integrity of the conductive path is also monitored by monitoring the electrical connection of the wrist strap to ground.
To achieve the aforementioned functional attributes, prior art wrist straps are often provided with a capacitance circuit for monitoring the electrical connection of the wrist strap to the user by monitoring the capacitance of the conductive line. As previously indicated, the capacitance of the conductive line is dependent upon the presence of a user, as well as the integrity of the conductive line connection to the user via the wrist strap. In addition to the capacitance circuit, the prior art wrist straps are often provided with a resistance circuit which monitors the electrical connection of the wrist strap to ground, thus insuring the presence of an adequate ground to which electrical charges are bled from the user. These prior art wrist bands, the user is typically alerted when electrical continuity through the wrist strap to ground is degraded. Thus, if any portion of the conductive path from the user to ground is not adequate to properly bleed off static electrical charge build-up upon the user, the user is alerted via a visual and/or audible alarm.
Though serving their intended purpose of bleeding static electrical charges from a user to ground, the prior art wrist bands possess certain deficiencies which detract from their overall utility. Though such wrist bands include circuits for monitoring the electrical connections of the wrist band to the user and the conductive line to ground, they do not include a circuit for monitoring the discharge resistor which, as previously indicated, is often incorporated in the conductive line to enhance the safety of the wrist strap. Additionally, the circuitry incorporated in the prior art wrist straps is extremely complex, thus causing the size of the wrist straps to be extremely large. The prior art circuitry also necessitates increased power input which significantly reduces the life of the battery incorporated into the wrist strap. Moreover, the prior art wrist straps, subsequent to being built, must be subjected to a complicated and time consuming test process which requires the use of a highly specialized text fixture.